1. Field of the Invention
The present invention relates to a method of cleaning a storage case to be used for storing or transporting mask substrates such as photomasks and photomask blanks, semiconductor substrates such as semiconductor wafers, pellicles, or the like.
2. Description of the Related Art
Recently, with the trend of the miniaturization and the high integration of semiconductor devices, wavelength of the light source of the exposing apparatus, used in photolithography techniques for the pattern formation has become shorter from the g line of a high pressure mercury lamp (436 nm) and the i line (365 nm), to the KrF excimer laser (248 nm), and the ArF excimer laser (193 nm). Since such exposing light sources of shorter wavelengths have high output with short wavelengths, the light energy is high, and thereby they cause a phenomenon where foreign substances are grown on photomasks or wafers used for the exposure as the time advances. It is pointed out that the growing of the foreign substances is more prominent as the wavelength of the exposing light becomes shorter. As the foreign substances generated on the photomasks become so large that they are transferred onto wafers, breakages of the circuit of the semiconductor devices or short circuit is caused, and thus it has been problematic.
As one of the major factors to generate the growing foreign substances on the photomasks at the time of using a short wavelength exposing light source, it is said that reaction of a sulfuric acid ion present on the mask surface as an acidic substance, and a basic substance such as ammonium present in the mask use-environment are accelerated by the excimer laser irradiation upon the pattern transfer so as to generate ammonium sulfide, or the like for providing the foreign substances (see, for example, Japanese Patent Application Laid Open Nos. 2006-11048, and 2004-53817).
In the semiconductor device production, reduction in defects of the photomasks or the semiconductor wafers is the essential task for ensuring a high yield. Accordingly, storage cases for storing cleaned photomasks with a purified surfaces or semiconductor wafers, and pellicles to be mounted on photomasks are mostly designed to prevent the stored subjects from being polluted with the dusts or the pollutants generated from the case constituent materials or the environment used therein.
For preventing the pollution of the above-mentioned mask substrates, the semiconductor substrates, or the pellicles with the ionic foreign substances, not only the mask substrates, the semiconductor substrates and the pellicles but also the storage cases should be cleaned regularly. As to the storage cases, in general, a method of washing by a wet process using such as hot water, a detergent-containing aqueous solution, or an acid is used. However, in the case of a storage case having a complicated shape, a problem arises in that the corner portion, or the like of the case cannot be washed sufficiently and it is hard to dry. Moreover, in the case a metal part is used for the case material, there is a problem in that acid, or the like cannot be used as the washing solution so that it is not easy to wash the case. Furthermore, since large scale equipments are required including the environmental countermeasure for the washing operation using a solvent, it leads to the production cost increase, and thus it has been problematic.
Therefore, in the case the shape of the storage case is complicated, or a metal part is used, since the cleaning operation by the conventional washing method is difficult, the storage case is used continuously as it is so as to lower the yield of the device production, or the case is replaced by a new one so as to bring about the production cost increase, and thus it has been problematic.